Ancillary services are essential for the safe, secure and reliable operation of power systems. They presently include both mandatory and competitive services such as voltage control, frequency control and system restart. Since these services were and are still mostly provided by centralized power plants, controls of voltage and frequency in the Turkish power system are also performed by power plants with installed capacity of 50 MW and greater. However, the number of private sector power plants connected to the national grid at medium voltage has been increasing continuously. The positive impact and the contribution of such plants should be realized better by further actions.